There is a continuing interest in the generation of hard copy from images created and/or stored in digitized form. Various devices have been designed for the output of such images in hard copy, such as ink-jet printers, thermal printers and laser scanners of various types. Laser scanners are particularly attractive output devices in view of their high resolution capability and the variety of different imaging media (e.g., both light-sensitive and heat-sensitive materials) that may be adapted for laser address.
Many heat-sensitive imaging media which are imageable by laser address comprise a photothermal converter, which converts laser radiation to heat, the heat being used to trigger the imaging process. IR-emitting lasers such as YAG lasers and laser diodes, are most commonly used for reasons of cost, convenience and reliability. Therefore, IR-absorbing dyes and pigments are most commonly used as the photothermal converter, although address at shorter wavelengths, in the visible region, is also possible as described in Japanese Patent Publication No. 51-88016.
Of particular interest are laser addressable thermal media giving rise to color images. Typically, such materials employ a donor sheet comprising a layer of colorant, which is placed in contact with a receptor, an IR absorber being present in one or both of the donor and receptor. Most commonly, the IR absorber is present only in the donor. When the assembly is exposed to a pattern of IR radiation, normally from a scanning laser source, the radiation is absorbed by the IR absorber, causing a rapid build-up of heat in the exposed areas, which in turn causes transfer of colorant from the donor to the receptor in those areas. By repeating the process with one or more different colored donors, a multi-color image can be assembled on a common receptor. The system is particularly suited to the color proofing industry, where color separation information is routinely generated and stored electronically and the ability to convert such data into hardcopy via digital address of "dry" media is seen as a great advantage.
The best-known of these systems are the various forms of thermal transfer imaging, including dye diffusion (or sublimation) transfer of a colorant without a binder (as described in U.S. Pat. No. 5,126,760), mass transfer of dyed or pigmented layers in a molten state (i.e., "melt-stick transfer" as described in JP 63-319192), and ablation transfer of dyes and pigments as a result of decomposition of binders or other ingredients to gaseous products causing physical propulsion of colorant material to the receptor (as described in U.S. Pat. No. 5,171,650 and WO90/12342). Other types of laser thermal color imaging media include those based on the formation or destruction of colored dyes in response to heat (U.S. Pat. No. 4,602,263), those based on the migration of toner particles into a thermally softened layer (WO93/04411) and various peel-apart systems wherein the relative adhesion of a colored layer to a substrate and a coversheet is altered by heat (WO93/03928, WO88/04237, and DE4209873).
A problem common to all of these media is the possibility of contamination of the final image by the laser absorber. For example, in the case of thermal transfer media, the absorber may be cotransferred with the colorant. Unless the cotransferred absorber has absolutely no absorption bands in the visible part of the spectrum, the color of the image will be altered. Various attempts have been made to identify IR dyes with minimal visible absorption (e.g., EP-A-0157568), but in practice the IR absorption band nearly always tails into the visible region, leading to contamination of the image.
A number of methods have been proposed to remove contamination by the absorber of the final image. For example EP-A-0675003 describes contacting the transferred image of laser thermal transfer imaging with a thermal bleaching agent capable of bleaching the absorber. This method complicates the imaging process and it has not been possible to bleach certain dyes, for example, CYASORB 165 (American Cyanamid) which is commonly used with YAG-lasers. WO93/04411 and U.S. Pat. No. 5,219,703 disclose an acid-generating compound which bleaches the IR absorbing dye. However, an additional UV exposure is generally required (optionally in the presence of a UV absorber), again complicating the imaging process. Thus, there is a continuing need for improved methods of bleaching the IR absorbing dye in laser addressed thermal media.
Photoredox processes involving dyes have been disclosed in the art. A photoexcited dye may accept an electron from a coreactant, the dye acting as a photo-oxidant. There are a number of examples where this type of process has been used, although not in the context of laser-addressable thermal imaging media. In particular, there are a number of systems comprising a cationic dye in reactive association with an organoborate ion (see U.S. Pat. No. 5,329,300, U.S. Pat. No. 5,166,041, U.S. Pat. No. 4,447,521, U.S. Pat. No. 4,343,891, and J. Chem. Soc. Chem. Commun., 299 (1993)). After transferring an electron to the excited dye, organoborate ions fragment into free radicals which may initiate polymerization reactions (J. Am. Chem. Soc., 110, 2326-2328 (1985)) or may react further and thus form an image (U.S. Pat. No. 4,447,521 and U.S. Pat. No. 4,343,891).
Another example of imaging involving photoreduction of a dye is disclosed in U.S. Pat. No. 4,816,379. This describes media comprising a photocurable layer containing a UV photoinitiator and photopolymerizable compounds, the layer additionally comprising a cationic dye of defined structure and a mild reducing agent capable of reducing said dye in its photoexcited state. Imagewise exposure at a wavelength absorbed by the cationic dye causes photoreduction of same and generation of a polymerization inhibitor, so that a subsequent uniform UV exposure gives polymerization only in the previously unexposed areas. Conventional wet development leaves a positive image. The cationic dyes are described as visible-absorbing, and are of a type not known to be IR-absorbing. Shifts in the absorbance of the cationic dyes (including bleaching) are noted. The preferred reducing agents are salts of N-nitrosocyclohexylhydroxylamine, but other possibilities include ascorbic acid and thiourea derivatives. There is no disclosure of thermal imaging media, however.
J. Imaging Sci. & Technol., 37, 149-155 (1993) describes the photoreductive bleaching of pyrylium dyes by allylthiourea derivatives under conditions of UV flood exposure. EP-A-O515133 and J. Org. Chem., 58, 2614-2618 (1993) disclose the photoreduction of neutral xanthene dyes by amines and other electron donors, for initiation of polymerization and in photosynthetic applications. The ability of dihydropyridine derivatives to transfer an electron to a photoexcited Ru(III) complex is disclosed in J. Amer. Chem. Soc., 103, 6495-6497 (1981). The reactions were carried out in solution and were not used for imaging purposes, however.
Thus, laser addressable thermal imaging media are still needed in which residual visible coloration from the laser absorber is minimized, and (in certain cases) in which crosslinking of the media is induced.